/sup 3/He(. pi. /sup -/,n)/sup 2/H and /sup 4/He(. pi. /sup -/,n)/sup 3/H at 50--295 MeV

The cross section of ..pi../sup -/+/sup 3/He..-->..d+n and ..pi../sup -/+/sup 4/He..-->..t+n has been measured at energies between T = 50 and 295 MeV and angles between theta/sub n/roughly-equal20/sup 0/ and 150/sup 0/ covering the momentum transfer range qroughly-equal0.4 --1.0 GeV/c. A phenomenological analysis of the data is attempted to identify the characteristic T, theta, and q dependences of the (..pi..,n) reaction dynamics. Most conspicuous is the angular dependence of the /sup 4/He(..pi../sup -/,n)/sup 3/He which mirrors characteristic features of ..pi..+He elastic scattering. The T dependence of (..pi..,n) in /sup 3,4/He (for theta<70/sup 0/) is similar to ..pi..+N or ..pi..+He elastic scattering. The clear dependence of T and theta is indicative of ..pi..+N (off-shell) scattering in the basic pion-absorption process. The nuclear form factor q dependence is most pronounced for forward angles, and the largely exponential falloff with q is similar to that of analogous nuclear reactions. Comparisons with theoretical calculations show that crucial ingredients are missing in currently used models and general directions of needed refinements are indicated.